This invention is directed to a method of cementing a casing string in an offshore wellbore to the wall of the wellbore from the bottom of the casing string to the seafloor.
In the drilling of a wellbore under a water body, such as an offshore wellbore under an ocean or sea, it is usually desirable to cement the first two strings of casing in the wellbore from the bottom of the casing string to the seafloor. These initial strings of casing may be referred to as "shallow" or "surface" casing strings and usually extend from the seafloor into the wellbore for several hundred to several thousand feet.
In the normal course of drilling an offshore wellbore, the initial portion of the wellbore is large and the walls thereof are irregular. In the cementing of a casing string in this initial portion of the wellbore it is desirable to cement the casing string from the bottom or shoe thereof to the seafloor. Because of the irregular walls of the wellbore, it is difficult or impossible to accurately determine the volume of the annular space or annulus formed intermediate the casing string and the wellbore wall. Therefore, in carrying out a cementing operation to cement the casing string in the wellbore, the volume of the annulus is often estimated and an excess volume of cement slurry is used to provide for filling the annulus from the casing shoe to the seafloor. The results of this approach are usually that either too little cement slurry is used and the annulus is not filled to the seafloor or too much cement slurry is used and the excess slurry flows out over the seafloor and is wasted.
The problem of cementing a casing string in an offshore wellbore and providing for the cement slurry to surround the casing string from the bottom of the casing string to the seafloor has been recognized and various techniques have been suggested for detecting when the returns of the cement slurry flowing up the annulus about a casing string reach the seafloor. One technique is to use divers who descend to the seafloor and watch for the cement returns during a cementing operation. This is difficult, dangerous and expensive and the results thereof are often unsatisfactory because of the difficulties in distinguishing the cement returns from the mud returns which flow from the annulus about the casing string. Television cameras have also been described as being positioned near the seafloor in the vicinity of a wellbore during a cementing operation to enable the observation of the cement returns at the seafloor but the same difficulties in recognizing the cement returns are encountered as there experienced by divers.
In U.S. Pat. No. 3,489,219 there is described a method of determining the level of a liquid hardenable material in the annular space between a borehole and a casing string whereby a detector is placed in the borehole and the liquid hardenable material is provided with a detectable material such as radioactive material and the detectable material is then detected by the detecting device. At least one liquid hardenable material detecting means is positioned within the borehole before the injection of the liquid hardenable material into the annular space is commenced and when the liquid hardenable material reaches the level of the liquid hardenable material detecting means a signal is transmitted to the surface or any accessible location which indicates that the liquid hardenable material has reached the vicinity of the detectors. Detecting means which may be employed include gamma detectors, x-ray detectors, G-M tube or ion chamber, a glass electrode or a hydrogen electrode, temperature-sensitive devices, detecting means which utilize any part of the electromagnetics spectrum, a capacitor, an electrical resistance or conductance, a detector sensitive to changes in the magnetic field, and electrodes.
In U.S. Pat. No. 2,524,933 there is described a method and apparatus for determining in a well the position of an interface between two fluids of dissimilar character. There is introduced into a well an interface detector that will distinguish physically or chemically between two immiscible fluids in the well. The detector is located in the region of the interface and alternately submerged in each of the two fluids. By this means a positive intermittent signal is transmitted to the surface which accurately indicates the position of the interface in the well. It is noted in this patent that means which had been suggested for detecting in a well the position of a static interface between two dissimilar fluids include electrical-conductivity cell, density meter, and means which employ the difference in the light-, sound-, and heat-transmitting characteristics of the fluids in a well.
In U.S. Pat. No. 2,171,840 an electrode unit is employed to determine the position of cement slurry in a wellbore. In U.S. Pat. No. 2,220,205 a method of locating cement in boreholes is disclosed wherein there is added to the cement a radioactive material and a gamma-ray detector is lowered in the well to determine the position of the cement containing radioactive material. In U.S. Pat. No. 2,453,456 a string of Geiger counters sensitive to gamma rays is suspended in a well and used for measuring the flow of fluid in a borehole. In U.S. Pat. No. 2,217,708 there is described a technique wherein an exothermic substance is placed within casing ahead of cement in a cementing operation and a thermometric means is used to locate the crest of the fluid cement by observing the marked temperature manifestations at only one point. In U.S. Pat. No. 2,050,128 there is described a technique for locating the top of cement behind well casing by running a temperature survey of the well.